In a multi-carrier communication system, such as an Orthogonal Frequency Division Multiple Access (OFDMA) system, the communicated signal consists of multiple subcarriers (also termed “tones”) that are designed to be mutually orthogonal when sampled at the right frequency points. Such orthogonality can be distorted by a number of factors, one of which is frequency error. In general, there are two potential sources of frequency error; namely, clock frequency error and Doppler shift. The clock frequency error is the difference in the clock frequency between a mobile device and its serving base station. Normally, the clock at the base station serves as the reference, to which the clock of a mobile device must be synchronized. The Doppler shift is caused by the movement of a mobile device relative to the base station, and the amount of shift depends on the speed and direction of the mobile device with respect to the base station.
The composite frequency error (i.e., the sum of all frequency errors including any clock frequency-error and Doppler shift) can be corrected if known. In the downlink (DL) case, the composite frequency error can be estimated based on the downlink signals and corrected by the receiver at the mobile device. In the uplink (UL) case, since the signals received by the base station consist of signals transmitted by multiple mobile devices, the composite frequency error is a mixture of frequency errors from different mobile devices. It may take a very complex process to mitigate the adverse effects of the combined errors at the base station. It would therefore be beneficial to develop an improved method of correcting for frequency errors in an environment with multiple mobile devices.